The present invention relates generally to masts used to support sails on wind powered boats, and more particularly to a sail boat having a mast which can be adjusted in angle with respect to the boat hull.
It will be appreciated by those skilled in the art that when sailboats are sailed in a configuration other than "running" before the wind, that is, when the wind is coming from behind the boat, they experience considerable side thrusts imposed by the force of the wind. These side thrusts exist when the boat is on a "reach", sailing perpendicular to the wind, or when it sails upwind, at an angle of as little as 30 degrees to the apparent wind angle. The apparent wind angle is the true wind angle, altered by the forward velocity of the boat.
Due to the side thrust imposed by the wind, the boat "heels", or leans away from the wind. Heeling results in three detrimental effects which impair the performance of a sailboat having a conventional mast system in which the mast remains perpendicular to the top deck of the boat hull. First, the weight of the mast, now displaced by the heel of the boat, creates a force or torque that increases the heel. Second, the slanted sail causes a downward force and an associated heeling torque as the thrust of the wind pushes down on the sail that is angled toward the leeward side of the boat. Third, the wind acting on the sail that is displaced by the heel of the hull creates an increase in the heeling force proportional to the displacement of the sail plan from over the center of buoyancy.
Several techniques have been used in the prior art to reduce the heel angle of sailboats, in an effort to make the boat "stiff" and bring the mast to a more vertical position with respect to the surface of the water. A near-vertical mast is well recognized to improve sailing performance, especially for upwind sailing. One technique has been to employ high ballast/displacement ratios, typical of race boats. The high technology racing boats use deep drafts with comparatively heavy keel bulbs, combined with lightweight hull construction. The result is a stiff rig. Unfortunately, such boats are not designed for comfortable cruising situations. The deep draft certainly restricts their use. Construction costs are high. When the boat heels, as it inevitably will, the mast is not vertical, so even high performance boats are detrimentally affected.
Other boat designers have tried movable water ballast to counteract heeling forces. This requires extensive and expensive plumbing. The water ballast is shifted slowly rendering it unsuitable except for relatively long reaches. In the eventuality that the ballast ends up on the leeward side, the results are less than desirable. There are other inherent design limitations associated with water ballast. For example assuming a boat having a 15 foot beam, and a ballast moment arm that is 7 feet long, then it would take approximately 450 cubic feet of water to create a 182,000 foot-pound righting moment to achieve a vertical mast for a typical fifty foot sailboat. This volume of water would weigh 28,000 pounds. This clearly would not work, and even a reduced water ballast results in significant increases in hull displacement and associated flow resistance.
Swing-keels have also been tried. But, assuming the use of a keel with all of the 14,000 pound keel weight in the keel bulb, and further assuming an ability to swing the keel 60 degrees in each direction, then it would take a keel that extended 13 feet below the center of buoyancy to result in a vertical mast for a typical fifty foot sailboat. Needless to say, such a boat would have an excessive draft. Furthermore, a keel swung 60 degrees would no longer function well to stabilize the hull while tracking upwind.
What is needed, then, is a sailboat which is capable of maintaining its mast in a substantially vertical position when the hull of the boat is heeling due to forces of the wind. Such a sailboat is not found in the prior art.